Flexible quantization

ABSTRACT

A digital media encoder/decoder uses a flexible quantization technique that provides the ability to vary quantization along various dimensions of the encoded digital media data, including spatial, frequency sub bands and color channels. The codec utilizes a signaling scheme to signal various permutations of flexible quantization combinations efficiently for primary usage scenarios. When a choice of quantizer is available, the codec efficiently encodes the current quantizer by defining a subset of quantizers and indexes the current quantizer from the set.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/418,690, filed May 5, 2006, which is hereby incorporated byreference.

BACKGROUND

1. Block Transform-Based Coding

Transform coding is a compression technique used in many audio, imageand video compression systems. Uncompressed digital image and video istypically represented or captured as samples of picture elements orcolors at locations in an image or video frame arranged in atwo-dimensional (2D) grid. This is referred to as a spatial-domainrepresentation of the image or video. For example, a typical format forimages consists of a stream of 24-bit color picture element samplesarranged as a grid. Each sample is a number representing colorcomponents at a pixel location in the grid within a color space, such asRGB, or YIQ, among others. Various image and video systems may usevarious different color, spatial and time resolutions of sampling.Similarly, digital audio is typically represented as time-sampled audiosignal stream. For example, a typical audio format consists of a streamof 16-bit amplitude samples of an audio signal taken at regular timeintervals.

Uncompressed digital audio, image and video signals can consumeconsiderable storage and transmission capacity. Transform coding reducesthe size of digital audio, images and video by transforming thespatial-domain representation of the signal into a frequency-domain (orother like transform domain) representation, and then reducingresolution of certain generally less perceptible frequency components ofthe transform-domain representation. This generally produces much lessperceptible degradation of the digital signal compared to reducing coloror spatial resolution of images or video in the spatial domain, or ofaudio in the time domain.

More specifically, a typical block transform-based codec 100 shown inFIG. 1 divides the uncompressed digital image's pixels into fixed-sizetwo dimensional blocks (X₁, . . . X_(n)), each block possiblyoverlapping with other blocks. A linear transform 120-121 that doesspatial-frequency analysis is applied to each block, which converts thespaced samples within the block to a set of frequency (or transform)coefficients generally representing the strength of the digital signalin corresponding frequency bands over the block interval. Forcompression, the transform coefficients may be selectively quantized 130(i.e., reduced in resolution, such as by dropping least significant bitsof the coefficient values or otherwise mapping values in a higherresolution number set to a lower resolution), and also entropy orvariable-length coded 130 into a compressed data stream. At decoding,the transform coefficients will inversely transform 170-171 to nearlyreconstruct the original color/spatial sampled image/video signal(reconstructed blocks {circumflex over (X)}₁, . . . {circumflex over(X)}_(n)).

The block transform 120-121 can be defined as a mathematical operationon a vector x of size N. Most often, the operation is a linearmultiplication, producing the transform domain output y=Mx, M being thetransform matrix. When the input data is arbitrarily long, it issegmented into N sized vectors and a block transform is applied to eachsegment. For the purpose of data compression, reversible blocktransforms are chosen. In other words, the matrix M is invertible. Inmultiple dimensions (e.g., for image and video), block transforms aretypically implemented as separable operations. The matrix multiplicationis applied separably along each dimension of the data (i.e., both rowsand columns).

For compression, the transform coefficients (components of vector y) maybe selectively quantized (i.e., reduced in resolution, such as bydropping least significant bits of the coefficient values or otherwisemapping values in a higher resolution number set to a lower resolution),and also entropy or variable-length coded into a compressed data stream.

At decoding in the decoder 150, the inverse of these operations(dequantization/entropy decoding 160 and inverse block transform170-171) are applied on the decoder 150 side, as show in FIG. 1. Whilereconstructing the data, the inverse matrix M⁻¹ (inverse transform170-171) is applied as a multiplier to the transform domain data. Whenapplied to the transform domain data, the inverse transform nearlyreconstructs the original time-domain or spatial-domain digital media.

In many block transform-based coding applications, the transform isdesirably reversible to support both lossy and lossless compressiondepending on the quantization factor. With no quantization (generallyrepresented as a quantization factor of 1) for example, a codecutilizing a reversible transform can exactly reproduce the input data atdecoding. However, the requirement of reversibility in theseapplications constrains the choice of transforms upon which the codeccan be designed.

Many image and video compression systems, such as MPEG and WindowsMedia, among others, utilize transforms based on the Discrete CosineTransform (DCT). The DCT is known to have favorable energy compactionproperties that result in near-optimal data compression. In thesecompression systems, the inverse DCT (IDCT) is employed in thereconstruction loops in both the encoder and the decoder of thecompression system for reconstructing individual image blocks.

2. Quantization

According to one possible definition, quantization is a term used for anapproximating non-reversible mapping function commonly used for lossycompression, in which there is a specified set of possible outputvalues, and each member of the set of possible output values has anassociated set of input values that result in the selection of thatparticular output value. A variety of quantization techniques have beendeveloped, including scalar or vector, uniform or non-uniform, with orwithout dead zone, and adaptive or non-adaptive quantization.

The quantization operation is essentially a biased division by aquantization parameter QP which is performed at the encoder. The inversequantization or multiplication operation is a multiplication by QPperformed at the decoder. These processes together introduce a loss inthe original transform coefficient data, which shows up as compressionerrors or artifacts in the decoded image. In a simplistic codec, acertain fixed value of QP can be applied to all transform coefficientsin a frame. While this may be an acceptable solution in some cases, ithas several deficiencies:

The human visual system is not equally sensitive to all frequencies, orto all spatial locations within a frame, or to all luminance andchrominance channels. Using different QP values for differentcoefficients may provide a visually superior encoding even with the sameor smaller number of compressed bits. Likewise, other error metrics canbe suitably optimized as well.

Rate control or the ability of an encoder to produce a compressed fileof a desired size is not easy to perform with a single QP across theentire frame.

It is therefore desirable to allow the encoder to vary QP across theimage in an arbitrary manner. However, this means that the actual valueof QP used for each data partition should be signaled in the bitstream.This leads to an enormous overhead just to carry the QP signalinginformation, making it unsuitable in practice. What is desired is aflexible yet bit-economic means of signaling QP, particularly forcommonly encountered scenarios.

In summary, quantization is the primary mechanism for most image andvideo codecs to control compressed image quality and compression ratio.Quantization methods supported by most popular codecs provide fewfeatures or little flexibility, or incur significant overhead ofadditional bits. Often, an image or a video frame is usually quantizeduniformly, or with limited ability to vary quantization over spatiallocations. This lack of flexibility hurts compression quality, andprevents accurate rate control on the fly. On the other hand, somecodecs provide nearly unrestricted freedom in supporting quantizationmethods. Encoding to signal use of different quantizers takes additionalbits in the encoded media, and could itself adversely affect compressionefficiency. Further, the process of building a conformant decoderrequires a large number of test passes generated by all possiblecombinations of the quantizer methods, which can be onerous.

SUMMARY

The following Detailed Description presents variations of a flexiblequantization technique that provides the ability to vary quantizationalong various dimensions of the encoded digital media data. For example,one representative implementation of the flexible quantization techniquecan vary quantization over three dimensions—over (i) spatial locations,(ii) frequency sub bands, and (iii) color channels. The DetailedDescription further presents ways to efficiently signal the flexiblequantization in the encoded digital media data. The benefit of thisquantization approach is that the overhead incurred by quantizationrelated side information is minimized for the primary usage scenarios,while allowing maximum flexibility if desired by the encoder.

This Summary is provided to introduce a selection of concepts in asimplified form that is further described below in the DetailedDescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. Additional features and advantages of the invention will be madeapparent from the following detailed description of embodiments thatproceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional block transform-based codecin the prior art.

FIG. 2 is a flow diagram of a representative encoder incorporating theblock pattern coding.

FIG. 3 is a flow diagram of a representative decoder incorporating theblock pattern coding.

FIG. 4 is a table containing a pseudo-code definition for signaling of aDC quantizer according to a flexible quantization technique.

FIG. 5 is a table containing a pseudo-code definition for signaling of alow-pass quantizer according to the flexible quantization technique.

FIG. 6 is a table containing a pseudo-code definition for signaling of ahigh-pass quantizer according to the flexible quantization technique.

FIG. 7 is a table containing a pseudo-code definition for signaling ofquantizers at a frame layer according to the flexible quantizationtechnique.

FIG. 8 is a table containing a pseudo-code definition for signaling ofquantizers at a tile layer in spatial mode according to the flexiblequantization technique.

FIG. 9 is a table containing a pseudo-code definition for signaling ofquantizers of a DC sub-band at the tile layer in frequency modeaccording to the flexible quantization technique.

FIG. 10 is a table containing a pseudo-code definition for signaling ofquantizers of a low-pass sub-band at the tile layer in frequency modeaccording to the flexible quantization technique.

FIG. 11 is a table containing a pseudo-code definition for signaling ofquantizers of a high-pass sub-band at the tile layer in frequency modeaccording to the flexible quantization technique.

FIG. 12 is a table containing a pseudo-code definition for signaling ofquantizers at a macroblock layer in spatial mode according to theflexible quantization technique.

FIG. 13 is a table containing a pseudo-code definition for signaling oflow-pass quantizers at the macroblock layer in frequency mode accordingto the flexible quantization technique.

FIG. 14 is a table containing a pseudo-code definition for signaling ofhigh-pass quantizers at the macroblock layer in frequency mode accordingto the flexible quantization technique.

FIG. 15 is a block diagram of a suitable computing environment forimplementing a media encoder/decoder with flexible quantization.

DETAILED DESCRIPTION

The following description relates to coding and decoding techniques thatprovide efficient flexible quantization that can vary the quantizationapplied along spatial, frequency and color dimensions (referred toherein as “Flexible Quantization”). The following description describesan example implementation of the technique in the context of a digitalmedia compression system or codec. The digital media system codesdigital media data in a compressed form for transmission or storage, anddecodes the data for playback or other processing. For purposes ofillustration, this exemplary compression system incorporating thisflexible quantization is an image or video compression system.Alternatively, the technique also can be incorporated into compressionsystems or codecs for other 2D data. The flexible quantization techniquedoes not require that the digital media compression system encodes thecompressed digital media data in a particular coding format.

1. Encoder/Decoder

FIGS. 2 and 3 are a generalized diagram of the processes employed in arepresentative 2-dimensional (2D) data encoder 200 and decoder 300. Thediagrams present a generalized or simplified illustration of acompression system incorporating the 2D data encoder and decoder thatimplement the block pattern coding. In alternative compression systemsusing the block pattern coding, additional or fewer processes than thoseillustrated in this representative encoder and decoder can be used forthe 2D data compression. For example, some encoders/decoders may alsoinclude color conversion, color formats, scalable coding, losslesscoding, macroblock modes, etc. The compression system (encoder anddecoder) can provide lossless and/or lossy compression of the 2D data,depending on the quantization which may be based on a quantizationparameter varying from lossless to lossy.

The 2D data encoder 200 produces a compressed bitstream 220 that is amore compact representation (for typical input) of 2D data 210 presentedas input to the encoder. For example, the 2D data input can be an image,a frame of a video sequence, or other data having two dimensions. The 2Ddata encoder divides a frame of the input data into blocks (illustratedgenerally in FIG. 2 as partitioning 230), which in the illustratedimplementation are non-overlapping 4×4 pixel blocks that form a regularpattern across the plane of the frame. These blocks are grouped inclusters, called macroblocks, which are 16×16 pixels in size in thisrepresentative encoder. In turn, the macroblocks are grouped intoregular structures called tiles. The tiles also form a regular patternover the image, such that tiles in a horizontal row are of uniformheight and aligned, and tiles in a vertical column are of uniform widthand aligned. In the representative encoder, the tiles can be anyarbitrary size that is a multiple of 16 in the horizontal and/orvertical direction. Alternative encoder implementations can divide theimage into block, macroblock, tiles, or other units of other size andstructure.

A “forward overlap” operator 240 is applied to each edge between blocks,after which each 4×4 block is transformed using a block transform 250.This block transform 250 can be the reversible, scale-free 2D transformdescribed by Srinivasan, U.S. patent application Ser. No. 11/015,707,entitled, “Reversible Transform For Lossy And Lossless 2-D DataCompression,” filed Dec. 17, 2004. The overlap operator 240 can be thereversible overlap operator described by Tu et al., U.S. patentapplication Ser. No. 11/015,148, entitled, “Reversible Overlap Operatorfor Efficient Lossless Data Compression,” filed Dec. 17, 2004; and by Tuet al., U.S. patent application Ser. No. 11/035,991, entitled,“Reversible 2-Dimensional Pre-/Post-Filtering For Lapped BiorthogonalTransform,” filed Jan. 14, 2005. Alternatively, the discrete cosinetransform or other block transforms and overlap operators can be used.Subsequent to the transform, the DC coefficient 260 of each 4×4transform block is subject to a similar processing chain (tiling,forward overlap, followed by 4×4 block transform). The resulting DCtransform coefficients and the AC transform coefficients are quantized270, entropy coded 280 and packetized 290.

The decoder performs the reverse process. On the decoder side, thetransform coefficient bits are extracted 310 from their respectivepackets, from which the coefficients are themselves decoded 320 anddequantized 330. The DC coefficients 340 are regenerated by applying aninverse transform, and the plane of DC coefficients is “inverseoverlapped” using a suitable smoothing operator applied across the DCblock edges. Subsequently, the entire data is regenerated by applyingthe 4×4 inverse transform 350 to the DC coefficients, and the ACcoefficients 342 decoded from the bitstream. Finally, the block edges inthe resulting image planes are inverse overlap filtered 360. Thisproduces a reconstructed 2D data output.

In an exemplary implementation, the encoder 200 (FIG. 2) compresses aninput image into the compressed bitstream 220 (e.g., a file), and thedecoder 300 (FIG. 3) reconstructs the original input or an approximationthereof, based on whether lossless or lossy coding is employed. Theprocess of encoding involves the application of a forward lappedtransform (LT) discussed below, which is implemented with reversible2-dimensional pre-/post-filtering also described more fully below. Thedecoding process involves the application of the inverse lappedtransform (ILT) using the reversible 2-dimensional pre-/post-filtering.

The illustrated LT and the ILT are inverses of each other, in an exactsense, and therefore can be collectively referred to as a reversiblelapped transform. As a reversible transform, the LT/ILT pair can be usedfor lossless image compression.

The input data 210 compressed by the illustrated encoder 200/decoder 300can be images of various color formats (e.g., RGB/YUV4:4:4, YUV4:2:2 orYUV4:2:0 color image formats). Typically, the input image always has aluminance (Y) component. If it is a RGB/YUV4:4:4, YUV4:2:2 or YUV4:2:0image, the image also has chrominance components, such as a U componentand a V component. The separate color planes or components of the imagecan have different spatial resolutions. In case of an input image in theYUV 4:2:0 color format for example, the U and V components have half ofthe width and height of the Y component.

As discussed above, the encoder 200 tiles the input image or pictureinto macroblocks. In an exemplary implementation, the encoder 200 tilesthe input image into 16×16 pixel areas (called “macroblocks”) in the Ychannel (which may be 16×16, 16×8 or 8×8 areas in the U and V channelsdepending on the color format). Each macroblock color plane is tiledinto 4×4 pixel regions or blocks. Therefore, a macroblock is composedfor the various color formats in the following manner for this exemplaryencoder implementation:

-   -   1. For a grayscale image, each macroblock contains 16 4×4        luminance (Y) blocks.    -   2. For a YUV4:2:0 format color image, each macroblock contains        16 4×4 Y blocks, and 4 each 4×4 chrominance (U and V) blocks.    -   3. For a YUV4:2:2 format color image, each macroblock contains        16 4×4 Y blocks, and 8 each 4×4 chrominance (U and V) blocks.    -   4. For a RGB or YUV4:4:4 color image, each macroblock contains        16 blocks each of Y, U and V channels.

Accordingly, after transform, a macroblock in this representativeencoder 200/decoder 300 has three frequency sub bands: a DC sub band (DCmacroblock), a low pass sub band (low pass macroblock), and a high passsub band (high pass macroblock). In the representative system, the lowpass and/or high pass sub bands are optional in the bitstream—these subbands may be entirely dropped.

Further, the compressed data can be packed into the bitstream in one oftwo orderings: spatial order and frequency order. For the spatial order,different sub bands of the same macroblock within a tile are orderedtogether, and the resulting bitstream of each tile is written into onepacket. For the frequency order, the same sub band from differentmacroblocks within a tile are grouped together, and thus the bitstreamof a tile is written into three packets: a DC tile packet, a low passtile packet, and a high pass tile packet. In addition, there may beother data layers.

Thus, for the representative system, an image is organized in thefollowing “dimensions”:

Spatial dimension: Frame→Tile→Macroblock;

Frequency dimension: DC|Low pass|High pass; and

Channel dimension: Luminance|Chrominance_0|Chrominance_1 . . . (e.g. asY|U|V).

The arrows above denote a hierarchy, whereas the vertical bars denote apartitioning.

Although the representative system organizes the compressed digitalmedia data in spatial, frequency and channel dimensions, the flexiblequantization approach described here can be applied in alternativeencoder/decoder systems that organize their data along fewer, additionalor other dimensions. For example, the flexible quantization approach canbe applied to coding using a larger number of frequency bands, otherformat of color channels (e.g., YIQ, RGB, etc.), additional imagechannels (e.g., for stereo vision or other multiple camera arrays).

2. Flexible Quantization Overview

In the representative encoder/decoder, the quantization operation isessentially a biased division by a quantization parameter QP which isperformed at the encoder. The inverse quantization or multiplicationoperation is a multiplication by QP performed at the decoder. However,alternative implementations of the flexible quantization describedherein can utilize other forms of quantization, including uniform andnon-uniform, scalar or vector, with or without dead zone, etc. Thequantization/inverse quantization processes together introduce a loss inthe original transform coefficient data, which shows up as compressionerrors or artifacts in the decoded image. In a simplistic codec, acertain fixed value of QP can be applied to all transform coefficientsin a frame. While this may be an acceptable solution in some cases, ithas several deficiencies:

The human visual system is not equally sensitive to all frequencies, orto all spatial locations within a frame, or to all luminance andchrominance channels. Using different QP values for differentcoefficients may provide a visually superior encoding even with the sameor smaller number of compressed bits. Likewise, other error metrics canbe suitably optimized as well.

Rate control or the ability of an encoder to produce a compressed fileof a desired size is not easy to perform with a single QP across theentire frame.

Ideally therefore, it should be possible to allow the encoder to vary QPacross the image in an arbitrary manner. However, this means that theactual value of QP used for each data partition(macroblock/tile/channel/sub band, etc.) should be signaled in thebitstream. This leads to an enormous overhead just to carry the QPsignaling information, making it unsuitable in practice. What is desiredis a flexible yet bit-economic means of signaling QP, particularly forcommonly encountered scenarios.

The flexible quantization technique described herein provides theability to vary quantization along various partitions or dimensions ofthe encoded digital media data. For example, one implementation of theflexible quantization technique in the representative encoder200/decoder 300 system can vary quantization over three dimensions—over(i) spatial locations, (ii) frequency sub bands, and (iii) colorchannels. However, quantization can be varied over fewer, additional orother dimensions or partitions of the data in other alternativeimplementations of the flexible quantization technique. This techniquealso includes ways to efficiently signal the flexible quantization inthe encoded media data. The benefit of this quantization approach isthat the overhead incurred by quantization related side information isminimized for the primary usage scenarios, while allowing maximumflexibility if desired by the encoder.

The flexible quantization technique provides fine spatial granularitycontrol of the quantization. In one particular implementation, theflexible quantization allows control over quantization applied to theframe, tile, or down to the macroblock. If the frame is not quantizeduniformly, then each tile can be quantized uniformly; if a tile is notquantized uniformly, then each macroblock will be quantized differently.

The flexible quantization further allows quantization control along thefrequency sub band dimension. In one particular implementation, theflexible quantization includes a sub band mode to specify a quantizationrelationship among frequency sub bands. The sub bands can be quantizeduniformly, or partially uniformly (low pass sub band using DC sub bandquantizer, and/or high pass sub band using low pass quantizer), orindependently.

The flexible quantization also allows control over quantization appliedalong the channel dimension of the data. In one particularimplementation, the flexible quantization includes a channel mode tospecify a quantization relationship among color channels. The channelscan be quantized uniformly, or partially uniformly (chrominance channelsuniformly but luminance independently), or independently.

The flexible quantization described herein also provides techniques toefficiently signal in side information of the compressed digital mediadata, combinations of the above quantization control over spatial,frequency sub band and channel that are of significance to the primaryusage scenarios. Further, the flexible quantization technique provides away to efficiently define choice of quantizer by indexing from a definedsubset of possible quantizers in the digital media data.

3. Flexible Quantization in the Spatial Dimension:

In the spatial dimension, three choices are provided by the flexiblequantization technique in the representative encoder/decoder:

-   -   The entire frame can be coded using the same quantization rule.    -   Else, an entire tile can be coded using the same quantization        rule and different tiles within the frame can use different        quantization rules.    -   Else, each macroblock within a tile can be coded using the same        quantization rule and different macroblocks within the tile can        use different quantization rules.

One means of signaling these possibilities is as follows: A binarysignal is sent in the bitstream at the frame level indicating whetherthe first possibility is true. If not, a fixed length symbol is sent inthe bitstream within each tile indicating the number of quantizationrules used for this tile. If the tile uses more than 1 quantizationrule, then a variable length symbol is sent within each macroblockwithin the corresponding tile that indicates the quantization rule usedby the macroblock. The decoder interprets the bitstream in a mannerconsistent with the encoder.

The representative encoder 200/decoder 300 uses a variant of the abovesignaling. A binary signal, represented by a generic syntax element,herein labeled as “XXX_FRAME_UNIFORM,” is only sent at the frame level(where XXX is a placeholder specifying the particular frequency sub bandor channel dimension of quantizer control). At the tile level, thenumber of distinct quantizer rules is sent in a tile-level syntaxelement (XXX_QUANTIZERS) only when the frame level syntax element(XXX_FRAME_UNIFORM) is false. If this number is equal to 1, it meansthat there is only one rule and therefore all macroblocks within thetile are uniformly coded with the same quantization rule (indicatingchoice 2), and if not it indicates choice of the third possibility.

4. Flexible Quantization Across Frequency Bands:

For flexible quantization across frequency bands, the bitstream syntaxof the representative encoder 200/decoder 300 defines two switches:

-   -   The low pass macroblock uses the same quantization rule as the        DC macroblock at the same spatial location. This corresponds to        the syntax element USE_DC_QUANTIZER.    -   The high pass macroblock uses the same quantization rule as the        low pass macroblock at the same spatial location. This        corresponds to the syntax element USE_LP_QUANTIZER.

These switches are enabled at the frame layer when the entire frame usesthe same quantization rule, or at the tile layer otherwise. Theseswitches are not enabled at the macroblock layer. All macroblocks withina tile therefore obey the same rules across frequency bands. A binarysymbol is sent for each of the switches at the appropriate (frame ortile) layer.

5. Flexible Quantization Across Image Channels:

For flexible quantization across channels, the bitstream syntax of therepresentative encoder 200/decoder 300 permits three choices:

-   -   All channels—luminance and chrominance have the same        quantization rule. This is indicated by the generic syntax        element XXX_CH_MODE==CH_UNIFORM.    -   Luminance follows one quantization rule and all chrominance        channels follow a different quantization rule, indicated by        XXX_CH_MODE==CH_MIXED.    -   All channels are free to choose different quantization rules,        indicated by XXX_CH_MODE==CH_INDEPENDENT.

6. Combinatorial Flexible Quantization:

The representative encoder 200/decoder 300 uses a bitstream syntaxdefined in the code tables shown in FIGS. 4-14 that can efficientlyencode the particular choice out of the flexible quantization optionsacross the dimensions discussed above. With several quantization optionsavailable across each of the spatial, frequency sub band and channeldimensions, the number of permutations of the available quantizationoptions is large. Adding to the complexity of flexible quantizationacross the three dimensions is the fact that the bitstream of therepresentative encoder 200/decoder 300 can be laid out in spatial orfrequency ordering. However, this does not change the availablequantization options, and only affects the serialization of the signals.The syntax defined in FIGS. 4-14 provides an efficient coding of thecombinatorial flexible quantization rules.

Some salient features of the combinatorial quantization rules as definedin the syntax of the representative encoder/decoder are as follows.

DC quantization is not allowed to vary on a macroblock basis. Thisallows the differential coding of quantized DC values without having todo an inverse scaling operation. Coding the DC band of an image tilewith a relatively small quantizer even when the AC (low pass and highpass) bands are coded with varying quantization does not appreciablyaffect the bit rate.

At one end of the scale, all transform coefficients within a frame usethe same quantization parameter. At the other end of the scale, low passand high pass quantization rules for all channels are allowed to varyindependently for each macroblock of the tile/frame. The onlyrestriction is that the number of distinct low pass and high passquantizer rules (covering all channels) is each restricted to 16. Eachsuch rule may specify independent values of quantization parameter foreach channel.

Between these extremes, several combinations are permitted as specifiedby the syntax tables shown in FIGS. 4-14.

7. Indexing of Quantizer Parameters:

The specific quantization parameter (QP) in the representativeencoder/decoder is based on a harmonic scale. An 8 bit value of aquantizer parameter index (QPI) corresponds to a value of QP which canbe relatively large. A second level of indexing is performed so thatQPIs varying across macroblocks can be coded in an efficient manner.

More particularly, the encoder 200 can define a set in the bitstreamcontaining between 1 and 16 QPI “vectors.” Each QPI vector is composedof one or more QPI values, based on which XXX_CHANNEL_MODE is chosen.Such sets are defined for DC, low pass and high pass sub bands, based onthe frequency band switch. Further, the DC set has only one QPI vectorsince only one DC quantizer is permissible in a tile-channel. The codingof these sets is defined in the tables shown in FIGS. 4-6.

As shown in the tables of FIGS. 7-11, signaling of the QPI vector setsof DC, low pass and high pass frequency sub bands occurs as follows.Based on the other coding modes, the cardinality of each set (i.e., thenumber of QPI vectors in the set) is indicated for low pass and highpass sub bands at the start of the corresponding tile or frame. Thecardinality of the DC set is 1. In the pseudo-code tables, the syntaxelement denoting cardinality is labeled as “XXX_QUANTIZERS.” (Inpractice, XXX_QUANTIZERS−1 is sent in the bitstream.) The syntaxelements labeled “XXX_QUANTIZER” in the tables denotes the coding of QPIsets, which is defined in the tables shown in FIGS. 4-6.

At the macroblock level, it is sufficient to send only the index QI ofthe desired QPI vector from within the QPI set. The tables in FIGS.12-14 define the syntax of sending QI on a macroblock basis. The syntaxelement corresponding to QI is labeled, “XXX_QUANTIZER_INDEX.” Avariable length code is used to signal QI. First, a one bit symbol issent indicating whether QI is zero or not. If not, then a fixed lengthcode of length being given by ceil(log₂(XXX_QUANTIZERS−1)) is sentindicating the specific QI different from zero. This allows for anefficient encoding of a “default” quantization rule (QI=0) with as lowas one bit per macroblock. When XXX_QUANTIZERS is 1, XXX_QUANTIZER_INDEXis uniquely zero and therefore QI need not be signaled.

8. Extensions

The above description of the flexible quantization is specific to itsimplementation in a representative encoder and decoder, and syntax.However, the principles of this technique are extensible to otherdigital media compression systems and formats as well. For instance, therepresentative encoder/decoder has only three frequency sub bands (DC,low pass and high pass). But, more generally, alternativeimplementations of the flexible quantization can be extended in astraightforward manner to a multitude of frequency sub bands. Likewise,alternative flexible quantization implementations can vary the quantizerat finer spatial granularity, such as by sending quantization index (QI)information at the sub-macroblock (such as block) level. Many extensionsto the underlying principles of the flexible quantization technique arepossible within the same framework.

9. Computing Environment

The above-described processing techniques for flexible quantization canbe realized on any of a variety of digital media encoding and/ordecoding systems, including among other examples, computers (of variousform factors, including server, desktop, laptop, handheld, etc.);digital media recorders and players; image and video capture devices(such as cameras, scanners, etc.); communications equipment (such astelephones, mobile phones, conferencing equipment, etc.); display,printing or other presentation devices; and etc. The flexiblequantization techniques can be implemented in hardware circuitry, infirmware controlling digital media processing hardware, as well as incommunication software executing within a computer or other computingenvironment, such as shown in FIG. 15.

FIG. 15 illustrates a generalized example of a suitable computingenvironment (1500) in which described embodiments may be implemented.The computing environment (1500) is not intended to suggest anylimitation as to scope of use or functionality of the invention, as thepresent invention may be implemented in diverse general-purpose orspecial-purpose computing environments.

With reference to FIG. 15, the computing environment (1500) includes atleast one processing unit (1510) and memory (1520). In FIG. 15, thismost basic configuration (1530) is included within a dashed line. Theprocessing unit (1510) executes computer-executable instructions and maybe a real or a virtual processor. In a multi-processing system, multipleprocessing units execute computer-executable instructions to increaseprocessing power. The memory (1520) may be volatile memory (e.g.,registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flashmemory, etc.), or some combination of the two. The memory (1520) storessoftware (1580) implementing the described digital mediaencoding/decoding with flexible quantization techniques.

A computing environment may have additional features. For example, thecomputing environment (1500) includes storage (1540), one or more inputdevices (1550), one or more output devices (1560), and one or morecommunication connections (1570). An interconnection mechanism (notshown) such as a bus, controller, or network interconnects thecomponents of the computing environment (1500). Typically, operatingsystem software (not shown) provides an operating environment for othersoftware executing in the computing environment (1500), and coordinatesactivities of the components of the computing environment (1500).

The storage (1540) may be removable or non-removable, and includesmagnetic disks, magnetic tapes or cassettes, CD-ROMs, CD-RWs, DVDs, orany other medium which can be used to store information and which can beaccessed within the computing environment (1500). The storage (1540)stores instructions for the software (1580) implementing the describeddigital media encoding/decoding with flexible quantization techniques.

The input device(s) (1550) may be a touch input device such as akeyboard, mouse, pen, or trackball, a voice input device, a scanningdevice, or another device that provides input to the computingenvironment (1500). For audio, the input device(s) (1550) may be a soundcard or similar device that accepts audio input in analog or digitalform from a microphone or microphone array, or a CD-ROM reader thatprovides audio samples to the computing environment. The outputdevice(s) (1560) may be a display, printer, speaker, CD-writer, oranother device that provides output from the computing environment(1500).

The communication connection(s) (1570) enable communication over acommunication medium to another computing entity. The communicationmedium conveys information such as computer-executable instructions,compressed audio or video information, or other data in a modulated datasignal. A modulated data signal is a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia include wired or wireless techniques implemented with anelectrical, optical, RF, infrared, acoustic, or other carrier.

The described digital media encoding/decoding with flexible quantizationtechniques herein can be described in the general context ofcomputer-readable media. Computer-readable media are any available mediathat can be accessed within a computing environment. By way of example,and not limitation, with the computing environment (1500),computer-readable media include memory (1520), storage (1540),communication media, and combinations of any of the above.

The described digital media encoding/decoding with flexible quantizationtechniques herein can be described in the general context ofcomputer-executable instructions, such as those included in programmodules, being executed in a computing environment on a target real orvirtual processor. Generally, program modules include routines,programs, libraries, objects, classes, components, data structures, etc.that perform particular tasks or implement particular abstract datatypes. The functionality of the program modules may be combined or splitbetween program modules as desired in various embodiments.Computer-executable instructions for program modules may be executedwithin a local or distributed computing environment.

For the sake of presentation, the detailed description uses terms like“determine,” “generate,” “adjust,” and “apply” to describe computeroperations in a computing environment. These terms are high-levelabstractions for operations performed by a computer, and should not beconfused with acts performed by a human being. The actual computeroperations corresponding to these terms vary depending onimplementation.

In view of the many possible embodiments to which the principles of ourinvention may be applied, we claim as our invention all such embodimentsas may come within the scope and spirit of the following claims andequivalents thereto.

We claim:
 1. A digital media processing method comprising: storing atleast part of a bitstream representing compressed digital media data ina memory; and decoding the at least part of the bitstream by applyingquantization parameters to transform coefficients of blocks of thedigital media data and applying inverse operations of a transform to theblocks of the digital media data, wherein the quantization parametersare selectively variable across at least a spatial dimension, afrequency dimension, and a color channel dimension, and wherein theselective variability across each of the dimensions is signaled bysignals in the bitstream.
 2. The method of claim 1, wherein the signalsinclude a signal indicating whether the same one or more quantizationparameters apply to an entire frame.
 3. The method of claim 1, whereinthe signals include a signal indicating whether the same one or morequantization parameters apply for DC coefficients and AC coefficients.4. The method of claim 1, wherein the signals include a signalindicating whether the same one or more quantization parameters applyfor luminance values and chrominance values.
 5. The device of claim 1,wherein the signals include a signal indicating whether the same one ormore quantization parameters apply to an entire frame.
 6. The device ofclaim 1, wherein the signals include a signal indicating whether thesame one or more quantization parameters apply for DC coefficients andAC coefficients.
 7. The device of claim 1, wherein the signals include asignal indicating whether the same one or more quantization parametersapply for luminance values and chrominance values.
 8. A digital mediaprocessing method comprising: storing at least part of a bitstreamrepresenting compressed digital media data in a memory; and decoding theat least part of the bitstream by applying quantization parameters totransform coefficients of blocks of the digital media data and applyinginverse operations of a transform to the blocks of the digital mediadata, wherein the quantization parameters applied to luminancecoefficients and to chrominance coefficients are selectively variablebetween at least a first chrominance quantization mode in which thechrominance coefficients use the same one or more quantizationparameters as the luminance coefficients and a second chrominancequantization mode in which the chrominance coefficients use differentquantization parameters than the luminance coefficients, whereinquantization parameters applied to DC coefficients and to ACcoefficients are selectively variable between at least a first frequencyquantization mode in which the AC coefficients use the same one or morequantization parameters as the DC coefficients and a second frequencyquantization mode in which the AC coefficients use differentquantization parameters than the DC coefficients.
 9. The method of claim8, wherein the first chrominance quantization mode and the secondchrominance quantization mode are signaled by a signal in the bitstream.10. The method of claim 8, wherein the quantization parameters areselectively variable between at least a first spatial dimensionquantization mode in which the same one or more quantization parametersare applied across all subdivisions of a frame and a second spatialdimension quantization mode in which subdivisions of the frame usedifferent quantization parameters.
 11. A picture decoder devicecomprising: a data storage buffer for storing at least part of abitstream of encoded picture data; and a processor programmed to decodethe at least part of the bitstream by applying quantization parametersto transform coefficients of blocks of the picture data according toquantization rules selectively applied according to signals in thebitstream and applying inverse operations of a transform to the blocksof the picture data, the signals in the bitstream including a firstsignal for determining how a quantization rule is to be applied across aspatial dimension, a second signal for determining how a quantizationrule is to be applied across a frequency dimension, and a third signalfor determining how a quantization rule is to be applied across a colorchannel dimension.
 12. The device of claim 11, wherein the first signalindicates whether or not the same one or more quantization parametersapply to an entire frame.
 13. The device of claim 11, wherein the secondsignal indicates whether or not the same one or more quantizationparameters apply for DC coefficients and AC coefficients.
 14. The deviceof claim 11, wherein the third signal indicates whether or not the sameone or more quantization parameters apply for luminance values andchrominance values.
 15. A picture decoder device comprising: a datastorage buffer for storing at least part of a bitstream of encodedpicture data; and a processor programmed to decode the at least part ofthe bitstream by applying quantization parameters to transformcoefficients of the blocks of the picture data and applying inverseoperations of a transform to the blocks of the picture data, wherein thequantization parameters applied to luminance coefficients and tochrominance coefficients are selectively variable between at least afirst chroma quantization mode in which the chrominance coefficients usethe same one or more quantization parameters as the luminancecoefficients and a second chroma quantization mode in which thechrominance coefficients use different quantization parameters than theluminance coefficients, wherein quantization parameters applied to DCcoefficients and to AC coefficients are selectively variable between atleast a first frequency quantization mode in which the AC coefficientsuse the same one or more quantization parameters as the DC coefficientsand a second frequency quantization mode in which the AC coefficientsuse different quantization parameters than the DC coefficients.
 16. Thedevice of claim 15, wherein the first chroma quantization mode and thesecond chroma quantization mode are signaled by a signal in thebitstream.
 17. The device of claim 15, wherein the quantizationparameters are selectively variable between at least a first spatialdimension quantization mode in which the same one or more quantizationparameters are applied across all portions of a frame and a secondspatial dimension quantization mode in which different portions of theframe use different quantization parameters.
 18. A digital mediaprocessing method comprising: encoding digital media data by applyingoperations of a transform to blocks of the digital media data andapplying quantization parameters to transform coefficients of the blocksof the digital media data, wherein the quantization parameters areselectively variable across at least a spatial dimension, a frequencydimension, and a color channel dimension, and generating a bitstream ofencoded digital media data, the bitstream further including signals forindicating how the quantization parameters were applied across thespatial dimension, the frequency dimension, and the color channeldimension.
 19. The method of claim 18, wherein the signals include asignal indicating whether the same one or more quantization parametersapply to an entire frame.
 20. The method of claim 18, wherein thesignals include a signal indicating whether the same one or morequantization parameters apply for DC coefficients and AC coefficients.21. The method of claim 18, wherein the signals include a signalindicating whether the same one or more quantization parameters applyfor luminance values and chrominance values.
 22. A picture encoderdevice comprising: a data storage buffer for storing picture data to beencoded; and a processor programmed to: encode digital media data byapplying operations of a transform to blocks of the digital media dataand applying quantization parameters to transform coefficients of theblocks of the digital media data, wherein the quantization parametersare selectively variable across at least a spatial dimension, afrequency dimension, and a color channel dimension, and generate abitstream of encoded digital media data, the bitstream further includingsignals for indicating how the quantization parameters were appliedacross the spatial dimension, the frequency dimension, and the colorchannel dimension.